1. Field of the Invention
The present invention relates to a color cathode ray tube, and more particularly, to a system for correcting an electron beam from a single cathode in an electron gun for a color CRT, in which the single electron beam emitted from the single cathode is controlled by a magnetic field generating device synchronous to an electron beam deflecting signal and a video signal applied to the cathode for realizing an optimal image.
2. Background of the Related Art
A general electron gun in a color cathode ray tube has three in-line cathodes for emitting electron beams. The three electron beams emitted from the cathodes are converged in the electron gun, and deflected by a non-uniform magnetic field having a self-convergence function from deflection yokes. Then, the electron beams pass through an electron beam pass through hole in a shadow mask, and reach to a panel having a coat of fluorescent material of red, green, and blue, to realize an image. The electron beams are involved in deterioration, such as mis-landing, mis-convergence, distortion, and change of spacing between the three electron beams due to the deflection yokes. If the screen is mad larger and flatter according to the trend of recent consumer's taste change, the deterioration of the electron beams becomes greater, degrading a quality of an image on the color cathode ray tube. Despite of the self-convergence function, the deflection yokes are involved in a longer distance of travel of the electron beams as the screen becomes larger, causing extents of the mis-convergence and the mis-landing greater. FIG. 1 illustrates an example of a mis-convergence of the electron beams wherein the solid lines represent mis-convergence and the dotted lines represent a right convergence, and FIG. 2 illustrates an example of mis-landings of the electron beams in the vicinities of the fluorescent materials wherein the solid lines represent the mis-landings and the dotted lines represent right landings. The distortion is caused by a disagreement between a deflection center of the deflection yokes and positions of the electron beams and an asymmetry of a deflection force. FIG. 3 illustrates an example of a green electron beam deviated to a fourth quadrant from the deflection center. The change of spacing between the three electron beams results in changes of incident angles of the three electron beams to the shadow mask as the electron beams are deflected because positions of the red, and blue electron beams are spaced from the deflection center. These changes of the spacings between the three electron beams passed through the shadow mask and reached to the panel, i.e., a spacing between the red beam and a green beam and a spacing between the green beam and a blue beam affect to a picture quality.
FIG. 4 illustrates a color purity magnet 10 mounted on an outside end of a neck portion for correcting the mis-landings and the mis-convergence of the electron beams before the electron beams pass through a magnetic field from the deflection yokes. The color purity magnet 10 is a device mounted for adjusting the electron beams by adjusting a relative position of a set of combinations of two annular magnets 10a, 10b and 10c each magnetized to have two poles, four poles, or six poles, for final compensation of an assembly error after assembly of the deflection yokes or the electron gun.
FIGS. 5A-5C illustrate examples of shifts of the electron beams caused by the set of combinations of two annular magnets 10a, 10b, 10c.
Referring to FIG. 5A, it can be known that a mis-landing of the electron beam to the green fluorescent material can be minimized if a two polar annular magnet 10a is adjusted. However, since the two polar annular magnet 10 also gives an influence both to the red, and the blue beams, the adjustment of a particular electron beam only is not possible in a case the three electron beams are present on the same time. And, because the set of combinations of two annular magnets 10a, 10b, and 10c of two, four or six poles are adjusted for an optimal electron beam landing of respective electron beams on a specific location of the panel before mounting on the neck, the electron beams can not be made to land at optimal positions for every deflection of the electron beams.